As one of the fuel economy improvement measures of an automobile triggered by global environment problems, weight reduction of the vehicle body is advancing, and it is necessary to high-strengthen a steel sheet used for an automobile as much as possible. However, when a steel sheet is high-strengthened for weight reduction of an automobile, elongation EL and r value (Lankford value) drop, and press formability and shape freezing property come to deteriorate.
In order to solve such problems, a hot-press forming method has been employed for manufacturing components in which a steel sheet is heated to a predetermined temperature (for example, a temperature at which a state of an austenitic phase is achieved), the strength is lowered (that is, forming is facilitated), the steel sheet is thereafter formed using a tool of a temperature (room temperature for example) that is lower compared with the case of a thin steel sheet, thereby impartation of a shape and a rapid heat treatment (quenching) utilizing the temperature difference of the both are executed simultaneously, and the strength after forming is secured.
According to such hot-press forming method, because forming is executed in a low strength state, spring back is also reduced (shape freezing property is excellent), a material added with alloy elements such as Mn, B and the like and having excellent quenchability is used, and thereby the strength of 1,500 MPa class in terms of the tensile strength is obtained by rapid cooling. Also, such hot-press forming method is referred to by various names such as a hot forming method, hot stamping method, hot stamp method, die quench method, and the like in addition to the hot-press method.
FIG. 1 is a schematic explanatory drawing showing a tool configuration for executing above-mentioned hot-press forming (may be hereinafter represented by “hot stamp”), 1 in the drawing is a punch, 2 is a die, 3 is a blank holder, 4 is a steel sheet (blank), BHF is a blank holding force, rp is punch shoulder radius, rd is die shoulder radius, and CL is punch/die clearance respectively. Also, out of these components, in the punch 1 and the die 2, passages 1a, 2a through which a cooling medium (water for example) can pass are formed inside of each, and it is configured that these members are cooled by making the cooling medium pass through these passages.
In hot stamping (hot deep drawing for example) using such tool, forming is started in a state the steel sheet (blank) 4 is heated to a single-phase zone temperature of Ac3 transformation point or above and is softened. That is, in a state the steel sheet 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3, the steel sheet 4 is pressed in to the inside of a hole of the die 2 (between 2, 2 of FIG. 1) by the punch 1, and is formed into a shape corresponding to the shape of the outer shape of the punch 1 while reducing the outside diameter of the steel sheet 4. Also, by cooling the punch 1 and the die 2 in parallel with forming, heat removal from the steel sheet 4 to the tools (the punch 1 and the die 2) is executed, holding and cooling are further executed at a forming bottom dead point (the temporal point the tip of the punch is positioned at the deepest point: the state shown in FIG. 1), and thereby quenching of the raw material is executed. By executing such forming method, a formed product of 1,500 MPa class with excellent dimensional accuracy can be obtained, the forming load can be reduced compared with a case a component of a same strength class is cold-formed, and therefore less capacity of the press machine is needed.
As a steel sheet for hot stamping widely used at present, one using 22Mn—B5 steel as a raw material is known. The steel sheet has the tensile strength of approximately 1,500 MPa and the elongation of approximately 6-8%, and is applied to a shock resistant member (a member not causing deformation as much as possible and not causing breakage in collision). Further, development of further high-strengthening (1,500 MPa or more, 1,800 MPa class) is also advancing by increasing the C content on the base of 22Mn—B5 steel.
However, the present situation is that a steel king other than 22Mn—B5 steel is scarcely applied, and a steel kind and a manufacturing method for controlling the strength and elongation of the component (for example, lowering the strength: 980 MPa class, elongation increasing: 20%, and the like) and widening the application range to other than shock resistant members are scarcely studied.
In a passenger car of the middle class or more, there is a case that both functions of a shock resistant portion and an energy absorption portion are secured within a component such as a B-pillar, rear side member, front side member and the like considering the compatibility in a side collision and a rear collision (a function for protecting the counterpart side also when a small-sized car collides with). In manufacturing the members described above, a method of laser-welding a high strength super high-ten of 980 MPa class and a ductile high-ten of 440 MPa class (tailored weld blank: TWB) for example and press-forming in a cold state has been a mainstream. However, recently, development of a technology for separately achieving the strength within a component by hot stamping is advancing.
For example, in non-patent literature 1, a method for hot stamping is proposed in which 22Mn—B5 steel for hot stamping and a material not achieving high strength even by quenching using a tool are laser-welded (tailored weld blank: TWB), and the tensile strength: 1,500 MPa (elongation: 6-8%) on the high strength side (shock resistant portion side) and the tensile strength: 440 MPa (elongation: 12%) on the low strength side (energy absorption portion side) are separately achieved. From a similar viewpoint, such technology as non-patent literature 2 has been proposed.
According to the technology of the non-patent literatures 1, 2, although the tensile strength is 600 MPa or less and the elongation is approximately 12-18% on the energy absorption portion side, laser-welding (tailored weld blank: TWB) is required beforehand, the number of the manufacturing steps increase, and the cost rises. Further, the energy absorption portion for which quenching is not required essentially comes to be heated which is not preferable from the viewpoint of calorie consumption also.
Furthermore, as a technology for separately achieving the strength within a component, such technologies as non-patent literatures 3, 4 for example have also been proposed. Out of them, according to the technology of the non-patent literature 3, the strength is separately achieved by making a blank a temperature difference (distribution) in a blank within a heating furnace, although 22Mn—B5 steel is a base, due to the effect of adding boron, the robust characteristic of the strength after quenching is inferior with respect to heating to a two-phase zone temperature, strength control on the energy absorption portion side is hard, and the elongation is only approximately 15%.
On the other hand, according to the technology of the non-patent literature 4, although the strength is separately achieved by changing the cooling rate within a tool (by heating a part of the tool by a heater, or by using materials with different thermal conductivity), 22Mn—B5 steel is a base, which is not rational in that the 22Mn—B5 steel which essentially has excellent quenchability is controlled so as not to be quenched (tool cooling control).